Here's why:
* Simple Predicate: The simple predicate is the verb (or verb phrase) that tells what the subject is doing or being.
* Subject: The subject is the noun or pronoun that performs the action or is described by the predicate. In this case, the subject is "Painting."
* Verb: The verb "is" connects the subject "Painting" to the predicate "an interesting form of relaxation".
Therefore, "is" is the simple predicate because it tells us what "Painting" is.